Light emitting semiconductor diodes (LEDs) are increasingly used for illumination purposes. An advantage of such light sources is their high efficiency and long life time.
The light emitting layer of a semiconductor which is for instance disposed as a film LED on a circuit board emits light within a wide angle of 180°. For many illumination purposes it is of advantage to bundle the light emitted by the LED(s). An angular range of about +/−30° from the central axis of the LED already includes about 70% of the emitted light power. However, it is desirable to collect the light in an emission angle as wide as possible such as to increase efficiency. In practice, special optics for this purpose are known which comprise a side reflector that reflects light which is emitted outside some angle, wherein the light is folded by another reflection and is then emitted in forward direction.
While such optics yield satisfying efficiencies, their fabrication is cumbersome due to the complex geometry which is required. Also, such complex geometries may generally be produced only from plastic material. Most plastic materials are not suitable for high power LEDs which may produce temperatures of 120° C. and more, since they are not sufficiently heat resistant. Also, plastics tend to haze, especially if exposed to high luminance for a long period.
Another way to collimate light emitted by a LED is by means of a Fresnel zone plate. Such diffractive optics is able to yield high efficiencies. However, the fabrication of micro-structures required for this, in particular in the area of wide emitting angles in which the spacing of the zone plate rings have to get thinner and thinner, is expensive and complex, unless impossible. Additionally, a Fresnel zone plate is only active for a single wavelength. To obtain a good collimation effect an array of Fresnel zone plates has to provide a separate zone plate for each LED color. White light LEDs which due to a converter material emit light in several wavelengths are not able to provided satisfying collimation by means of a Fresnel zone plate.
Prior micro-lens arrays made of glass which conventionally are fabricated employing thin film techniques, generally fail to provide a satisfying collimation effect, at least in the peripheral area of the emitted light.